[FEATURE] Add attachments to mail queue

Closes #3

See merge request !9

Enable the mail queue to store attachments and send them later

We use a PlaintextService to convert the HTML body to a plaintext
version of it to be able to send it alongside the mail

[BUGFIX] Reset template for current domain only

See merge request !8

If a mail is sent via the mail queue, we must also set the sending_time
to prevent the scheduler command controller from sending the same mail
that has been send via the mail queue again.

Add the marker replacement to the subject, so markers can be used
inside the subject, too

If subject is a string that begins with LLL:, it will be handled
as a locallang reference and translated accordingly

The only arguments to be sent should be the required ones:
selectedExtension and selectedTemplate to prevent the redirect url
from becoming to long for some web servers to handle